1. Field of the Invention
The present invention relates to a signal processing device for processing an image signal and outputting the processed image signal into an electrophotographic printer such as a laser printer, an LED printer, etc.
2. Description of the Prior Art
In general, when a host computer outputs an image signal into an output unit such as a laser printer, etc., there are provided two types of image signals, which are a code information, such as a character code, or a dot data of the image. In the printer using the code information, the bit map method or the character map method are generally used. The printer using a bit map method can print out any characters, such as graphic characters, at a desirable position, however, a bit map printer has a cost disadvantage since the printer can not pause during printing, making it necessary for the printer to have a memory capacity sufficient for at least one printed of page data. On the other hand, if the host computer outputs an image signal of dot data the printer at a rate equal to or greater than the printing speed of the printer, the printer can print out the inputted data without pausing. However, the output speed of the host computer is generally lower than the printing speed of the printer and it is necessary for the printer to print out independent of the transferring speed. An electrophotographic printer has a predetermined process speed that depends on the sensitivity of the photoconductor body of the printer, the developing and transferring characteristics etc. Therefore, it is necessary to provide a large capacity buffer memory in the printer for storing the dot data corresponding to one printed page, resulting in the printer being more expensive.
In order to decrease the memory capacity in the printer, an encoding method for compressing the image data is generally used. That is, the data from the host computer is encoded by a predetermined encoding method, resulting in compression of the data. Then, after the compressed data is stored in a buffer memory having a capacity smaller than the aforementioned buffer memory arranged in the printer, the compressed data stored in the memory is decoded and outputted in the printer in synch with the printing speed of the printer.
A decoding method of the image signal is suggested in the Japanese provisional publication (Kokai) No. 58-170280, and the decoding method will be described below, referring to FIG. 1.
FIG. 1 is a schematic diagram showing the decoding method of the image signal. In FIG. 1, raster data outputted from a raster data generator 20 is first compressed by a compressing unit 21. Then, the compressed data is outputted in series into a shift register 23, the shift register 23 latches the inputted serial data bit by bit synchronizing with a clock shift signal outputted from a shift clock generator 22, and the shift register 23 outputs the compressed data in parallel into a ROM 24. The compressed parallel data is decoded by the ROM 24 and the decoded data outputted into a latch 25 of the printer.
When n bits of data are outputted from the compressing unit 21 into the shift register 23, it is necessary for the shift register 23 to shift the inputted data n times and n clock pulses are required to latch on data of the image signal. Therefore, there is a problem since the ROM 24 can not output the data synchronizing with the printing speed of the printer, since it takes a long time for the shift register 23 to latch the serial data of the image signal.